If the 'proof' that the sai originated in China is records of what appeared to be an elongated sai there before sais were used in Okinawa, a trident is like an elongated sai, and there are plenty of referrences to it going back to many years BC elswhere in the world. Perhaps it's origins are in neither China or Okinawa, or perhaps the basic design has multiple origins. They were used by roman gladiators and feature in ancient mythology too.